Read here. Most official thermometers recording daily temperatures are nowadays located in urban settings (airports are popular settings) where concrete, asphalt and metal raise the surrounding area temperature. As a result, global warming anomalies reported by the official climate agencies are biased warmer, which are never correctly adjusted to remove the urban heat island impact (bias).
This chart from 'The Inconvenient Skeptic' is a current example of the 'urban heat island' effect on temperatures.
